Flatter diurnal cortisol slopes amplify the association between daily stress and somatic symptoms

Daily somatic symptoms, such as fatigue and headaches, are common among adults and can forecast health decline. Flatter diurnal cortisol slopes (DCS), indicating a dysregulated hypothalamic-pituitary-adrenal (HPA) axis, are linked to physical health impairments. Whether DCS moderate the within-person relationship between daily stressors and somatic symptoms, and whether stress buildup further influences this association, remain unclear.

Middle-aged community adults (N = 1621) completed daily diary interviews and offered salivary cortisol samples over four days, producing 6484 person-day observations. Three-level generalized linear mixed-effects models using a Tweedie distribution examined main effects, two-way and three-way interactions among DCS, within-person daily stressor exposure, and stress-buildup burden on daily somatic symptom counts. Daily stress was assessed for both stressor frequency and severity; the average stressor severity was assigned a priori as the key definition.

Within-person stressor exposure was linked to higher same-day somatic symptoms across all definitions. Flatter DCS were associated with greater somatic symptom burden when stress was defined as the daily stressor count (β = 0.217). DCS significantly moderated the count-derived stress-symptom relationship (β = 0.019) but did not moderate the severity-average relationship (β = -0.017). Moreover, stress buildup moderated this cortisol-related pattern (three-way interaction: β = -0.043), but this outcome was sensitive to the imputation approach and should be interpreted with caution.

DCS was related to differential somatic reactivity to everyday stress, such that flatter DCS was connected with stress-somatic symptom coupling. Contextual stressors and physiological regulation likely jointly affect daily somatic health among adults.